You see that 1m22.952s on mediums from Bottas is already fairly impressive - so no doubt the Mercedes is working well.
Consider that fastest medium tyre time last year was 1:21.609 and it looks very good. We should bear in mind a couple of caveats to that though:
- this year's medium is last year's soft (fastest time for that tyre last year was 1:19.267)
- the track changes to get rid of bumps is said to be worth up to a second
Still, it's these kinds of indications that, to me, gives a sense of whether teams are looking generally strong (and, no surprise, Mercedes are). Not to mention the fact that he's out there straight away putting the laps in.